Growth rates of water droplets were measured with a static diffusion cloud condensation chamber in May–June 2007 at a rural field site in Southern Ontario, Canada, 70 km north of Toronto. The observations include periods when the winds were from the south and the site was impacted by anthropogenic air from the U.S. and Southern Ontario as well as during a 5-day period of northerly wind flow when the aerosol was dominated by biogenic sources. The growth of droplets on anthropogenic size-selected particles centred at 0.1 μm diameter and composed of approximately 40% organic and 60% ammonium sulphate (AS) by mass, was delayed by on the order of 1 s compared to a pure AS aerosol. Simulations of the growth rate on monodisperse particles indicate that a lowering of the water mass accommodation coefficient from α_<i>c</i>=1 to an average of α_<i>c</i>=0.04 is needed (assuming an insoluble organic with hygroscopicity parameter, κ_org, of zero). Simulations of the initial growth rate on polydisperse anthropogenic particles agree best with observations for α_<i>c</i>=0.07. In contrast, the growth rate of droplets on size-selected aerosol of biogenic character, consisting of &gt;80% organic, was similar to that of pure AS. Simulations of the predominantly biogenic polydisperse aerosol show agreement between the observations and simulations when κ_org=0.2 (with upper and lower limits of 0.5 and 0.07, respectively) and α_<i>c</i>=1. Inhibition of water uptake by the anthropogenic organic applied to an adiabatic cloud parcel model in the form of a constant low α_<i>c</i> increases the number of droplets in a cloud compared to pure AS. If the α_<i>c</i> is assumed to increase with increasing liquid water on the droplets, then the number of droplets decreases which could diminish the indirect climate forcing effect. The slightly lower κ_org in the biogenic case decreases the number of droplets in a cloud compared to pure AS.